The present invention relates to a method for locating defects in varistors that are apt to cause device failure, the method involving incipient hot spot observation.
Varistors are used in conjunction with expensive electrical power distribution systems to protect these systems from overvoltages or from surges, that is, extremely brief, extremely high overvoltages, due to a lightning discharge, for example. Varistors typically comprise a ceramic body, such as zinc oxide, together with small amounts of other metal oxide additives, and a pair of opposing electrodes situated on either side of the ceramic body.
Varistors have occasionally failed in operation, particularly when subject to surge conditions, permitting damage to occur to expensive power distribution equipment. Various methods are known for detecting defects in varistors that are likely to cause varistor failure under operating conditions. One known approach, generally, is to estimate the amount of energy a varistor device can absorb, which is simply the maximum attainable product of device voltage, device current, and duration of current flow, without device failure. This approach yields a useful quality control measure of device performance, but is apt to overlook varistor defects that are only fatal to the device during surge conditions.
A second, known approach of detecting defects in varistors, as exemplified by U.S. Pat. No. 4,112,362 to Hower et al., is to form on one side of a varistor ceramic body a temporary electrode structure comprising multiple, discrete electrical contacts and to form a conventional, permanent electrode on another side of the varistor body. A current is pulsed through each contact, one at a time. Varistor current level measurements are made to provide data upon which device performance can be predicted. This approach is inherently limited to a low testing current level since an attempt to pass a high level of current through each discrete contact would result in varistor surface breakdown at the respective edges of the discrete contacts due to high electric fields at such edges. The latter prior art approach, therefore, can only provide an extrapolated estimate of device performance under high current conditions because of inability to simulate such current conditions.